One of the niche applications for fiber optics is as a sensor for “downhole” applications, such as monitoring a geothermal well, oil well, or the like. Downhole measurements permit the operator to monitor multiphase fluid flow, as well as pressure and temperature. Downhole measurements of pressure, temperature and fluid flow play an important role in managing various types of sub-surface reservoirs.
Historically, the monitoring systems have been configured to provide an electrical line that allows the measuring instruments, or sensors, to send measurements to the surface. Recently, fiber optic sensors have been developed that communicate readings from a wellbore to optical signal processing equipment located at the surface. The fiber optic sensors may be variably located within the wellbore. For example, optical sensors may be positioned on the outer surface of a submersible electrical pump and used to monitor the performance of the pump. Fiber optic sensors may also be disposed along the tubing within a wellbore. In either instance, a fiber optic cable is run from the surface to the downhole sensing apparatus. The fiber optic cable transmits optical signals to an optical signal processor at the surface which is then used to determine environmental information (such as temperature and/or pressure) associated with the wellbore.
With respect to geothermal wells, a fiber optic sensor may be used to obtain a temperature profile along the depth of the well. It is well known in the art that a vertical temperature profile of an entire geothermal well can be obtained essentially instantaneously using a single optical fiber. Inasmuch as the intensity of various frequency components of backscattered light within the optical fiber depend upon the temperature of the medium at the point where the backscattered light is generated, proper detection and analysis of the entire backscattered radiation spectrum will yield the desired temperature profile.
However, field tests of optical fiber distributed temperature sensors have demonstrated that conventional optical fibers are insufficiently robust for this type of application. In “hot” well studies, anomalies associated with changes in the optical transmission characteristics of the studied optical fibers began to appear within the first twenty-four hours of the test period. Inasmuch as it is desired to deploy these optical fiber sensors for long periods of time, this type of change is unacceptable.
At least a portion of the anomalies have been associated with the formation of OH ions (and other hydrogen-related moieties) in the silicate glass matrix of the optical fibers. The OH ions do not exist in the optical fiber prior to its exposure to the “downhole” environment. The likely degradation mechanism is that hydrogen in the hot downhole environment diffuses into the fiber, and within the fiber the hydrogen reacts with the oxygen of the silicate glass to form OH ions.
The constituents of the glass have been found to have a strong influence on the rate at which OH ions are formed in a typical downhole environment. Optical fibers typically have a core glass with a refractive index value that is greater than the refractive index value of a surrounding cladding glass, so as to maintain confinement of the propagating optical signal within the core area. An optical fiber may have what is referred to as a “step-index” structure, where there is essentially an abrupt interface between the core and cladding glasses, or alternatively, a “graded-index” structure, where there is a gradual change in refractive index in a radial direction from the center of the core. It is common, in either case, to introduce germanium into the core area to increase its refractive index. It has been found, however, that the presence of germanium promotes the formation of OH ions in the downhole environment.
Thus, a need remains in the art for an effective optical fiber sensor for downhole applications that remains stable within a hydrogen-rich environment, even at elevated temperatures.